North-facing rooms receive indirect, relatively cool and consistent daylight with no direct sun — colours can read flatter and cooler than in a sample book or a well-lit shop.
Hemisphere note: This describes typical Northern Hemisphere daylight. In the Southern Hemisphere, north-facing rooms typically get the strong, warm direct sun described here for south-facing rooms — the physical mechanism is the same, but which compass direction gets the sun is reversed.
Warm-undertone colours generally cope better, because they counteract the room's cool light rather than compounding it. Colours with a definite hue also hold up better than near-neutrals: there is enough of their own colour present that the light's cast is a small proportion of what you see. Deeper colours are frequently a better answer than pale ones here — a common instinct is to go as pale as possible to compensate for the lack of light, but a pale colour has little to reflect and often just reads as grey.
Cool greys and blues can look noticeably flatter and greyer than expected — a colour that looked crisp under bright shop lighting can look dull and lifeless once painted. Near-neutrals are the highest-risk group of all: beige can turn pink or slightly green, and a warm grey can lose its warmth entirely. Brilliant white is also worth avoiding here, since it has no warmth to counteract the light and typically reads as cold and blue.
Testing advice
Test a physical sample on the actual wall and view it at several times of day, including under heavy overcast, which is the room at its least flattering. Test on more than one wall: even without direct sun, the wall opposite the window receives noticeably more light than the wall containing it. Judge near-neutrals in threes rather than singly, because their casts are only visible in comparison.
Lighting simulations and descriptions here are approximations, not exact physical predictions — see methodology.